What causes Feline Hyperthyroidism?

Hyperthyroidism means your cat has over-active thyroid glands: they’re producing too much of the thyroid hormones T3 and T4.

This happens when your cat’s thyroid glands have become enlarged. Both glands are usually affected, although one may be more enlarged.

Hyperthyroidism is the most common hormone-related condition in older cats; the vast majority of cats diagnosed with hyperthyroidism will be over 10 years old. Very rarely, young cats are also affected.

What are the symptoms of Feline Hyperthyroidism?

Hyperthyroidism causes a steady progression of worsening symptoms, and can lead to secondary complications which will also need treating.

If you think your cat might have this condition, or you’ve already had a diagnosis from your vet, you might recognise the following symptoms:

Losing weight despite (usually) a ravenous appetiteOvereating – but occasionally alossof appetiteDrinking and urinating more (litter trays will be wetter)VomitingHyperactivity, restlessness, nervousness – sometimes irritability or aggressionDeterioration in your cat’s coat; may appear oily and unkemptDiarrhoeaIn severe cases, severe muscle loss or heart damage causing loss of strengthGetting out of breath too quickly.

How is Feline Hyperthyroidism Diagnosed?

In many cases, the diagnosis of feline hyperthyroidism is by a straightforward blood test. This tests the levels of thyroid hormone in the blood, and if the level is high then the diagnosis is made. In some cases, the diagnosis can be more challenging however, because other illnesses can cause thyroid levels to decrease, meaning cats with the disease are missed. 

What happens if I don't treat my cat?

If hyperthyroidism isn’t diagnosed and treated it will progress:

Your cat is likely to experience progressively severe weight loss – even if he or she is over-eating.Your cat is very likely to develop secondary conditions as a result of the hyperthyroidism.   These can include serious heart problems which can result in heart failure.  High blood pressure can also be encountered in hyperthyroid cats and this  can lead to damage of  your cat’s eyes, kidneys, heart and brain.

What is the treatment for Feline Hyperthyroidism?

There are many treatment options for feline hyperthyroidism. The majority of cases are started initially on an oral medication (tablet or liquid), which stops the production of the thyroid hormone. This does not cure the disease, rather it controls it so the symptoms and consequences of the disease resolve. However, as it does not sure the disease, treatment is required for the rest of the patient’s life.

After initial stabilisation with oral medications, a number of curative treatments are also available. These include surgical removal of the thyroid gland (thyroidectomy) or treatment with radioactive iodine. Radioactive iodine targets the thyroid tissue – as this is the only tissue in the body that uses significant quantities of iodine. The radioactive material therefore builds up in the thyroid tissue and destroys it, without causing damage to other tissues within the body. Radioactive iodine administration can only be performed at specialist facilities due to the use of a radioactive substance, and a period of hospitalisation is required after treatment is given to reduce environmental exposure.

Other, less commonly used treatments include ultra-low iodine diets, which prevent thyroid hormone production by eliminating the supply of iodine (required to make it), and trans-dermal gels (creams containing medication that are absorbed through the skin). Although some effect is seen with these treatments, the effectiveness is very variable and they are challenging to accurately dose, therefore they are not typically utilised unless other methods are exhausted.